In downhole drilling devices according to the prior art, power for driving downhole electric components has normally been supplied via wires from the surface or battery packs included in the downhole device. Power may be required for guiding the drill bit in the well while drilling, so as to be able to drill deviated wells. Power transfer via long wires is undesirable because it limits the design options for the drilling device as long power-transferring wires cannot connect to both rotating and non-rotating parts of the drilling device. Power is also lost in the wire during transfer. Power supplied from included battery packs limits the lifetime between runs to pull out and replace the battery packs, and may significantly increase the length and running cost of the device.
Downhole power sources, such as mud generators, have been used for downhole power generation, and mud generators offer the advantage of downhole sustainable power supply. However, such mud generators are typically provided in the hollow rotating drill string, rotating together with the drill string. Existing power transfer devices generally rely on mechanical contact devices, such as slip-ring devices, which are prone to damage, wear and high contact resistance and which become less efficient over time. The consequence of this is high power losses and temperature increases. No robust solution exists for transferring power from the continuously rotating drill string to nonrotating parts of the bottom hole assembly. On the other hand, various forms of accumulators, such as batteries, could be provided on non-rotating parts, but the power from an accumulator is limited as is the space for placing such accumulators downhole.